The present invention generally relates to the manufacture of plate blocks for lead batteries which are comprised of a plurality of positive and negative plates provided with plate lugs which are interconnected by bridges (pole straps) cast around the plate lugs in a casting mold.
The combination of plates to form a plate block is ordinarily accomplished by stacking positive and negative grid plates in an alternating sequence, with separators placed between them, so that the lug conductors of the positive plates and the lug conductors of the negative plates are positioned on top of, and along opposite sides of the resulting stack. The assembled plate block is then inverted so that the lugs hang downwardly, for immersion in casting molds which face toward the plate block. The casting molds are filled with molten lead for forming the pole bridges which are to connect the plates of the same polarity. As a result, the lugs are fused onto and are thus connected to the cast lead mass which, after hardening, forms the pole bridges of the plate block. The casting molds are arranged at a fixed distance from one another so that both bridges can be cast simultaneously.
In practice, it has been found that these operations can result in poor connections between the plate lugs and the cast-on bridges. A poor connection can break when the battery is used, or can exhibit excessive electrical resistance. It has been found that this results from the thin oxide layer which conventionally covers the lug surfaces, and which represents an obstacle to coalescence of the poured-in lead and the (heated) lead forming the plate lugs. For this reason, the plate lugs are frequently immersed in a flux solution before casting in order to remove the surface oxides and improve the contact which results.
The overall conditions important for a good cast-on operation include good heat transfer balanced with energy (heat) economy. On the one hand, the heat carried away from the lead melt (e.g., by the lugs) should not be too great. Otherwise, hardening of the lead melt will take place too rapidly. On the other hand, additional heating of the lead melt is to be avoided for reasons of economy. Efforts have been made to optimize this process, but have not been entirely satisfactory.
For example, U.S. Pat. No. 4,160,071 discloses plate lugs which, instead of having the normally rectangular cross-section, are reduced to a wedge-shape toward their outer end. By this shaping, not only is the mass of the lug kept to a minimum, but unnecessary heating (and the resulting cost) is prevented. However, because of this shaping, and due to the wettability of molten lead, the molten lead (despite a low immersion depth) tends to rise on the wedge profile, forming a meniscus. As a result, the part of the lug which is not immersed becomes filled with adhering lead, so that after hardening, the lug takes on the usual rectangular cross-section in the tapered connection zone and in the untapered part.
To produce the same melting and cooling conditions for all plate lugs, DE-OS 36 20 891 suggests that the geometry of the casting molds be changed in such a way that the amount of heat accumulating on each lug (given, in each case, by the volume and the heat capacity of the bridge-forming lead associated with the lug and the surrounding segment of the mold wall) is essentially of the same magnitude. The system is constructed as if it were separated into longitudinal segments (each including one lug) which have the same (or identical) heat content. This condition is met by providing a casting mold in which the lead volume of the bridge is reduced, and the surrounding mold walls are thickened, in proportion to the plate lugs.
However, in battery manufacture, it is common for the positive and negative plates to be constructed so that the plates of one polarity have thicker plates lugs than the plates of the other polarity. However, when fabricated into common plate blocks, for simultaneous connection of the two sets of plates with their lead bridges, only the same casting molds can be used or are available.